Jean M. Twenge's article is so important that I am going to summarize her findings here.

First off, you should know that Ms. Twenge is a professor of psychology at San Diego State University and the author of Generation Me and iGen.

Twenge has been researching generational differences for 25 years. She writes: "Typically, the characteristics that come to define a generation appear gradually, and along a continuum. Beliefs and behaviors that were already rising simply continue to do so...

However, around 2012, she noticed abrupt shifts in teen behaviors and emotional states, "...many of the distinctive characteristics of the Millennial generation began to disappear." In all her analyses of generational data—some reaching back to the 1930s—she had never seen anything like it.

She asked herself, "What happened in 2012 to cause such dramatic shifts in behavior? [I realized]...it was exactly the moment when the proportion of Americans who owned a smartphone surpassed 50 percent."

She identifiest iGen as "...a generation shaped by the smartphone and by the concomitant rise of social media... Born between 1995 and 2012, members of this generation are growing up with smartphones, have an Instagram account before they start high school, and do not remember a time before the Internet. The Millennials grew up with the web as well, but it wasn’t...at hand at all times... iGen’s oldest members were early adolescents when the iPhone was introduced, in 2007, and high-school students when the iPad entered the scene, in 2010..."

She believes "...the impact of these devices has not been fully appreciated, and goes far beyond the usual concerns about curtailed attention spans."

It's not all bad: "...today’s teens are physically safer than teens have ever been. They’re markedly less likely to get into a car accident and, [have] less of a taste for alcohol than their predecessors...The teen birth rate hit an all-time low in 2016, down 67 percent since its modern peak, in 1991."

"Psychologically, however, they are more vulnerable than Millennials were: Rates of teen depression and suicide have skyrocketed since 2011. It’s not an exaggeration to describe iGen as being on the brink of the worst mental-health crisis in decades."

"The allure of independence, so powerful to previous generations, holds less sway over today’s teens, who are less likely to leave the house without their parents. The shift is stunning: 12th-graders in 2015 were going out less often than eighth-graders did as recently as 2009.

Today’s teens are also less likely to date...only 56 percent of high-school seniors in 2015 went out on dates; for Boomers and Gen Xers, the number was about 85 percent.

The decline in dating tracks with a decline in sexual activity. The drop is the sharpest for ninth-graders, among whom the number of sexually active teens has been cut by almost 40 percent since 1991. The average teen now has had sex for the first time by the spring of 11th grade, a full year later than the average Gen Xer.

Nearly all Boomer high-school students had their driver’s license by the spring of their senior year; more than one in four teens today still lack one at the end of high school...In conversation after conversation, teens described getting their license as something to be nagged into by their parents—a notion that would have been unthinkable to previous generations.
Beginning with Millennials and continuing with iGen, adolescence is contracting again—but only because its onset is being delayed. Across a range of behaviors—drinking, dating, spending time unsupervised— 18-year-olds now act more like 15-year-olds used to, and 15-year-olds more like 13-year-olds. Childhood now stretches well into high school.
Why are today’s teens waiting longer to take on both the responsibilities and the pleasures of adulthood? ...iGen teens have more leisure time than Gen X teens did, not less.
...So what are they doing with all that time? They are on their phone, in their room, alone and often distressed.
The number of teens who get together with their friends nearly every day dropped by more than 40 percent from 2000 to 2015; the decline has been especially steep recently. It’s not only a matter of fewer kids partying; fewer kids are spending time simply hanging out.

You might expect that teens spend so much time in these new spaces because it makes them happy, but most data suggest that it does not. The Monitoring the Future survey, funded by the National Institute on Drug Abuse and designed to be nationally representative, has asked 12th-graders more than 1,000 questions every year since 1975 and queried eighth- and 10th-graders since 1991.

The survey asks teens how happy they are and also how much of their leisure time they spend on various activities, including nonscreen activities such as in-person social interaction and exercise, and, in recent years, screen activities such as using social media, texting, and browsing the web. The results could not be clearer: Teens who spend more time than average on screen activities are more likely to be unhappy, and those who spend more time than average on nonscreen activities are more likely to be happy.There’s not a single exception.The portrait of iGen teens emerging from the data is one of a lonely, dislocated generation.

One piece of data that indirectly but stunningly captures kids’ growing isolation, for good and for bad: Since 2007, the homicide rate among teens has declined, but the suicide rate has increased. As teens have started spending less time together, they have become less likely to kill one another, and more likely to kill themselves. In 2011, for the first time in 24 years, the teen suicide rate was higher than the teen homicide rate.

What’s the connection between smartphones and the apparent psychological distress this generation is experiencing? ...social media ... exacerbate the age-old teen concern about being left out...
This trend has been especially steep among girls. Forty-eight percent more girls said they often felt left out in 2015 than in 2010... ...Social media levy a psychic tax on the teen doing the posting as well, as she anxiously awaits the affirmation of comments and likes.

These more dire consequences for teenage girls could also be rooted in the fact that they’re more likely to experience cyberbullying...Social media give middle- and high-school girls a platform on which to carry out the style of aggression they favor, ostracizing and excluding other girls around the clock.
iGeners sleep with their phones.. They check.. social media right before they go to sleep, and reach for their phone as soon as they wake up in the morning...

The smartphone is also cutting into teens’ sleep...Fifty-seven percent more teens were sleep deprived in 2015 than in 1991.

The correlation between depression and smartphone use is strong enough to suggest that more parents should be telling their kids to put down their phone.

This will be challenging - Twenge notes "I’ve observed my toddler, barely old enough to walk, confidently swiping her way through an iPad."

Twenge concludes her article on a somewhat optimistic note: "I saw hopeful signs that kids themselves are beginning to link some of their troubles to their ever-present phone."

I wasn't convinced.

This is something all of us, every generation, is going to have to think about and address.